Heretofore, as shown in Frantz U.S. Pat. No. 3,890,122, issued June 17, 1975, filter units in compressed air systems have passed air to a reservoir usually through an outlet check valve or sometimes through an unvalved outlet line, the latter if the unit is open to atmosphere only briefly during the compressor's idling cycle. In either case, the objective is to prevent the reservoir from losing pressure through the filter unit when the compressor is not pumping. Unless the ambient air is exceptionally dry or the filter unit contains a desiccant, the air will be saturated or even super-saturated with moisture on both entering the leaving the unit, although the outgoing air will have a lower absolute moisture content, if, as in the Frantz patent, the filter unit is adapted to remove moisture other than by adsorption.
The problem posed by a filter unit of the non-desiccant type in passing at least saturated air to a reservoir, whether through a check valve or an unvalved outlet, is that the resistance to flow is substantially or entirely the back pressure from the reservoir. In turn, the compressor, through a governor, is responsive in its pumping and idling cycles to reservoir pressure, idling when the reservoir reaches a predetermined maximum or cutout pressure and beginning to pump at a predetermined minimum or cut-in reservoir pressure and continuing to pump until the pressure in the reservoir is again at maximum. Thus, during the pumping cycle, the saturated air passing from the filter unit will range in pressure from at or slightly above the minimum to the maximum reservoir pressure. The absolute moisture content of saturated air decreasing with increasing pressure, the inevitable result is that, when the reservoir is fully charged to its maximum pressure, the air in the reservoir will be super-saturated, to the detriment of air brakes or other equipment operated by reservoir pressure. It is with a solution of this problem that the present invention is primarily concerned.